U.S. Pat. No. 7,399,438 of Clark et al., which is incorporated herein by reference, describes methods of manufacturing lignocellulosic composite materials and doors made of a frame structure covered by thin-layers of such composite materials known as door skins. The composite materials and door skins may be made by mixing wood fiber, wax, and a resin binder, and then pressing the mixture under conditions of elevated temperature and pressure to form a thin-layer wood composite that is then bonded to the underlying door frame or core. As described in the '438 patent, composite door skins are conventionally formed by pressing wood fragments between heated dies in the presence of a binder at temperatures exceeding 275° F. (135° C.). The resin binder used in the door skin may be an isocyanate-based resin, a formaldehyde-based resin, a thermoplastic resin, or a thermoset resin.
A significant problem in the manufacture of wood-based composite products that are exposed to the outdoor environment and extreme interior environments is that upon exposure to variations in temperature and moisture, the wood can lose water and shrink, or gain water and swell. This tendency to shrink and/or swell can significantly limit the useful lifetime of most exterior wood products, such as wooden doors, often necessitating replacement after only a few years. The problem is particularly prevalent in extremely wet climates and extremely hot or dry climates. Door skins made of a composite mixture of wood fibers, fiberglass, and a resin binder have recently been introduced in the market, which provide improved resistance to moisture. Composite materials and door skins made of fiberglass and resin and without any cellulosic fiber content are also known.
The '438 patent describes a process utilizing isocyanate-based resins instead of formaldehyde-based resins to yield lignocellulosic fiber composite door skins having increased resistance to changes in environmental moisture. Isocyanate-based resins may also provide environmental benefits over formaldehyde-based resins. However, the present inventors have found that it is more difficult in some respects to make composites with isocyanate-based resins than with formaldehyde-based resins. For example, isocyanate-based resins have a greater tendency to adhere to the working surfaces of the steel dies used for pressing the composite mixture. This tendency can lead to a build-up of resin or composite material on the die surface, which causes undesirable defects in the surface finish of door skins.
The '438 patent describes several generally complementary approaches to inhibiting adhesion and build-up on die surfaces, including the use of an internal release agent in the composite mixture, the application of a release agent on the surface of a mat of the composite mixture prior to pressing the mat, and the application of anti-bonding agents on the die surface. Some of the various anti-bonding agents described in the '438 patent involve coating the die surface with a liquid composition that is baked into the die to form a stable anti-bonding coating that can be used for 2000 press cycles. The '438 patent also describes that the use of a release agent and/or an anti-bonding agent during the manufacture of cellulosic composite door skins may allow for increased resin content in the composite, which may improve the strength and surface finish of door skins. Notwithstanding the use of anti-bonding agents on the dies and release agents in or on the composite mixture, a build-up will eventually form on the dies over the course of many successive pressing cycles, requiring the dies to be regularly removed from the press for cleaning and recoating with the anti-bonding agent. Removal and recoating of the dies leads to equipment downtime, added expense, and waste.
Accordingly, a need exists for improved means and methods of preventing composite adhesion to and build-up on the dies used for pressing door skins and other composite materials.